1. Field of the Invention
The invention relates to nuclear steam generators for nuclear power plants and more particularly to a shimming system including a structure and a method for fixing existing steam generators with the potential for excessive heat exchange tube damage from vibration.
2. Prior Art
An earlier method of expanding flattened tubes to reduce vibration amplitudes in nuclear steam generators is the subject of U.S. Pat. No. 4,624,304, assigned to the same assignee as the instant invention.
U.S. Pat. Nos. 4,640,342 and 4,653,576 relate to expensive expandable anti-vibration bar arrangements for use in nuclear steam generators having anti-vibration bars located between the U-shaped tubes and attached at their ends to wrap-around bars or retainer rings on the outside of the tube bundle in the area of the tube bends.
The wear problem that exists due to the anti-vibration bar type of heat exchanger is in the upper heat exchange tube bundle between the tubes and the anti-vibration bars or tube supports which separate columns of pairs of tubes. These upper tube supports have an original clearance with pairs of tubes between which they pass. Thus, there is a given distance x, typically 0.359 inches, between the pairs of tubes and a width y, typically 0.340 inches, of the anti-vibration bar which leaves room, typically 0.19 inches, for tube vibration amplitude.
The purpose of the anti-vibration bars is to limit the movement of the tubes when subjected to flow induced vibration that tends to occur when two phase flow exists on the exterior of the tubes. The anti-vibration bars were designed to limit the amount of motion of the tubes. It is apparent that the designers of this type of steam generator were striving to limit the cyclic bending stresses on the tubes and minimize the possibility of tubes impacting against each other. Unfortunately, in the commercial embodiment in use, the utilization of anti-vibration bars with a typical nominal clearance of 0.0095 inches between each anti-vibration bar and adjacent tube has resulted in excessive wear of the tubes during service. In some instances, this design has required that the tubes be plugged to prevent a primary coolant passage to the secondary side of the heat exchanger, through tube walls, during operation of the steam generator.